This invention relates to novel plastic compositions having enhanced environmental degradability.
The advent of plastics has given rise to improved methods of packaging goods. For example, polyethylene and polypropylene plastic films, bags, bottles, styrofoam cups and blister packages have the advantages of being chemically resistant, relatively unbreakable, light in weight and translucent or transparent. The increasing use of plastics in packaging applications has created a serious waste disposal problem. Burning of these plastic materials is unsatisfactory since it adds to air pollution problems.
Unlike some other packaging materials, such as paper and cardboard, plastics are not readily destroyed by the elements of nature. Thus, burying them is not an effective means of disposal, and can be expensive.
Plastics are biologically recent developments, and hence are not easily degradable by microorganisms which attack most other forms of organic matter and return them to the biological life cycle. It has been estimated that it may take millions of years for organisms to evolve which are capable of performing this function. In the meantime, plastic containers and packaging films are beginning to litter the countryside after being discarded by careless individuals.
One approach to the alleviation of the problem of plastics waste and litter would involve the development of novel polymeric compositions which undergo accelerated degradation under environmental conditions. This general approach has been described in the prior art. For example, reference is made to British Patent Specification No. 1,128,793, which describes ethylenecarbon monoxide copolymers which undergo rapid deterioration when subjected to sunlight. Presumably, such materials undergo photolytic chain scission as a result of the absorption of ultraviolet light by the ketone group derived from carbon monoxide.
Photosensitizing ketone groups have also been introduced into the structure of polymer molecules by the copolymerization of ethylenically unsaturated monomers with vinyl ketones. For example, reference is made to German Offen. Patent No. 2,119,855, which describes the preparation of photo-degradable polymeric materials, e.g., by the copolymerization of methyl methacrylate and methyl vinyl ketone.
Polymers having ketone groups within the polymer chain, or at positions adjacent to the chain, such as those derived from carbon monoxide or vinyl ketone co-monomers, are believed to undergo photochemical degradation by direct photolytic chain scission, without the formation of reactive intermediates such as free radicals. This mechanism does lead to a rapid decrease in molecular weight upon exposure to ultraviolet radiation. However, if we consider such materials as polymeric additives to be blended with unmodified organic polymers, the "direct photolysis" mechanism causes the modified polymer to be an inefficient sensitizer. In other words, when a blend of modified and unmodified molecules is exposed to ultraviolet radiation, the modified molecules undergo degradation but the unmodified molecules remain unaffected.
This invention is intended to provide photo-degradable polymeric materials which are also effective photosensitizing additives when combined with unmodified polymers. This is accomplished by the introduction of photosensitizing ketone groups into positions which are not adjacent to the atoms of the molecular chain, in such a manner that the ketone groups, upon photo-excitation, tend to form free radical intermediates, and thus promote photo-oxidative degradation rather than photolytic chain scission.
The processes of this invention also differ from those of the prior art in another respect, namely that the photosensitive polymer is prepared by chemical modification of a previously synthesized polymer, rather than by copolymerization of a ketone-containing monomer with another vinyl monomer.